1. Field of the Invention
The present invention generally relates to moisture sensors and, in particular, to carbon nanotube sensors for repeated detection of relative humidity, and methods for operation thereof.
2. Brief Description of the Background Art
Conventional moisture sensing systems include a Loop Powered Dewpoint Transmitter (LPDT), such as the HTF™ Al2O3 sensor provided by Xentaur Corporation. The LPDT system allows a user to view dewpoint by monitoring the operation of a sensor constructed as a capacitor having a dielectric that consists of porous aluminum oxide, as well as monitoring the gas that enters pores of the aluminum oxide. The capacitor plates, i.e. electrodes, are an aluminum substrate and a porous gold layer deposited on top of the aluminum oxide, and the electrode having the porous gold layer allows transfer of gases into or out of the aluminum oxide pores. The capacitance due to the aluminum oxide is constant, while the capacitance due to the gas will vary according to the gas content and pressure. Since the dielectric constant of water is orders of magnitude larger than the dielectric constant of any gas being measured, the quantity of water vapor present in the pores will change the capacitance of the sensor to a much greater extent than other variables.
Dewpoint from the LPDT system can be obtained from Equation (1):
                              D          =                                                                      (                                      I                    -                    4                                    )                                ×                                  (                                      H                    -                    L                                    )                                            16                        +            L                          ,                            (        1        )            
where I is the current drawn by the LPDT, H is a value of the high end of an analog output range, L is a value of a low end of the analog output range, and D is the measured dewpoint. However, use of the LPDT sensor will often cause the pore volume to change due to contaminant clogging, residual oxidation, metal migration, etc. Thus, recalibration of the sensor is needed, based on a capacitance at a wet end of the dewpoint-capacitance curve, such as shown in FIG. 1.
Another conventional system is described in U.S. Pat. No. 6,865,940 to Poole, which is incorporated herein by reference. Poole utilizes an aluminum oxide moisture sensor that requires several hours to reach equilibrium levels that vary by application process.
Conventional ice sensors include a control unit and a sensor probe, which contains a control unit monitored with two parallel, redundant optical beams with corresponding detectors. See, e.g. Aerosonde Robotic Aircraft Report dated Sep. 21, 2000, Barrow August 2000 Operation: Icing Sensor Data Report by Dan Thomas. The control unit will monitor intensity of the beams to detect ice buildup. The parallel beams are used to protect against false readings of ice accretion. The sensor probe also contains temperature sensors used by the control unit logic to verify that the air temperature is below a threshold temperature, at which temperature icing can occur. However, ice layers accumulate very quickly, making early detection of ice formation imperative in applications such aircraft safety.